1. Field of the Invention
The present invention relates to a method of producing a negative electrode for use in a lithium secondary cell.
2. Description of the Background Art
A solid secondary cell with a thin lithium film has been proposed. Japanese Patent Laying-Open No. 62-44960 discloses a method for manufacturing such a solid cell. The method includes successively forming a thin film of titanium disulfide as a positive electrode, a thin film of Li2Oxe2x80x94Al2O3 as an electrolyte, and a thin film of Li as a negative electrode on a substrate placed in an ionized cluster beam evaporation system. Japanese Patent Publication No. 5-48582 discloses an electrolytic material for such a solid cell.
On the other hand, advances have been made in commercialization of lithium secondary cells containing an organic solution of electrolytes. Lithium secondary cells are characterized by having a high-energy output per unit area or per unit weight as compared with other cells. Lithium secondary cells have been developed for practical use as a power source in mobile communications equipment, notebook computers, electric vehicles and the like.
An attempt has been made to use lithium metal for a negative electrode for the purpose of improving the performance of the lithium secondary cell containing an organic solution of electrolytes. Such an attempt, however, involves the risk of a dencdroid growth of the lithium metal during charging and discharging. The dendroid growth may form an internal short-circuit to a positive electrode and finally result in an explosion. As a technique for avoiding the risk, an attempt can be made to form a thin film of a sulfide-based inorganic solid electrolyte on the lithium metal.
The lithium metal, the thin film of the sulfide-based inorganic solid electrolyte, and the source materials therefor, however, are highly reactive to water, so that they cause the problem of degradation when exposed to the air. The above-mentioned publications related to the solid cell, however, do not suggest a technique for independently producing a lithium-containing negative electrode itself. The problem of the degradation described above must be resolved for the production of such a freestanding negative electrode containing lithium and the sulfide-based solid electrolyte.
An object of the present invention is to provide a method of producing a negative electrode for a lithium secondary cell, in which lithium metal, a source material for a thin film of a sulfide-based inorganic solid electrolyte, and a negative electrode having the thin film of the inorganic air. solid electrolyte formed thereon can be prevented from being degraded by
The present invention is directed to a method of producing a negative electrode for a lithium secondary cell having a thin film made of an inorganic solid electrolyte. The method includes using a negative electrode base material placed in a closed container and an inorganic solid electrolyte source material placed in a closed container. The negative electrode base material has a surface made of a material selected from the group consisting of lithium metal and lithium alloys. In the method, the negative electrode base material placed in the closed container and the inorganic solid electrolyte source material placed in the closed container are placed into a chamber space, which is substantially inactive to lithium and which is insulated from air and provided adjacent to an apparatus for forming a thin film. In the chamber space, the negative electrode base material and the source material are respectively taken out from the closed containers. Then, the negative electrode base material and the source material taken out are transferred into the apparatus for forming a thin film without being exposed to the air. In the apparatus, the materials are used, and a thin film made of an inorganic solid electrolyte is formed on the negative electrode base material. The negative electrode base material having the thin film formed thereon is then transferred without being exposed to the air into a chamber space, which is substantially inactive to lithium and which is insulated from the air and provided adjacent to the apparatus. In the chamber space, the negative electrode base material having the thin film is placed into a closed container. The negative electrode having the thin film placed in the closed container can be taken out from the chamber into the air without being degraded.
The negative electrode base material for use in the method may be prepared by forming a thin film made of a material selected from the group consisting of lithium metal and lithium alloys on a base material by vapor deposition. The thin film made of the material selected from the group consisting of lithium metal and lithium alloys preferably has a thickness of 20 xcexcm or less. The thickness of the thin film is typically in the range of 0.1 xcexcm to 20 xcexcm, and preferably in the range of 1 xcexcm to 10 xcexcm.
The present invention is directed to another method of producing a negative electrode for a lithium secondary cell having a thin film made of an inorganic solid electrolyte. The method includes using a first source material placed in a closed container and a second source material placed in a closed container. The first source material is selected from the group consisting of lithium metal and lithium alloys. The second source material is for use in forming the inorganic solid electrolyte. The first and second source materials respectively placed in the closed containers are placed into a chamber space, which is substantially inactive to lithium and which is insulated from air and provided adjacent to an apparatus for forming a thin film. In the chamber space, the first and second source materials are respectively taken out from the closed containers. Then, the first and second source materials taken out are transferred into the apparatus without being exposed to the air. In the apparatus, the first and second materials are used, and a first thin film made of the first source material and a second thin film made of the second source material are formed on a base material. The base material having the first and second thin films formed thereon is then transferred without being exposed to the air into a chamber space, which is substantially inactive to lithium and which is insulated from the air and provided adjacent to the apparatus. In the chamber space, the base material having the first and second thin films is placed into a closed container. The negative electrode having the thin films placed in the closed container can be taken out from the chamber into the air without being degraded.
In the method, the first thin film may be formed by a vapor deposition method. The first thin film preferably has a thickness of 20 xcexcm or less. The thickness of the thin film formed is typically in the range of 0.1 xcexcm to 20 xcexcm, and preferably in the range of 1 xcexcm to 10 xcexcm.
The present invention is directed to a further method of producing a negative electrode for a lithium secondary cell having a thin film made of an inorganic solid electrolyte. In the method, a first source material selected from the group consisting of lithium metal and lithium alloys is placed in a closed container, and the first source material placed in the container is placed into a chamber space, which is substantially inactive to lithium and which is insulated from air and provided adjacent to a first apparatus for forming a thin film. In the chamber space, the first source material is taken out from the closed container. Then, the first source material taken out is transferred into the first apparatus without being exposed to the air. In the first apparatus, the first source material is used, and a first thin film made of the first source material is formed on a base material. The base material having the first thin film formed thereon is transferred from the first apparatus without being exposed to the air into a chamber space, which is substantially inactive to lithium and which is insulated from the air and provided adjacent to the first apparatus. In the chamber space, the base material having the first thin film formed thereon is placed into a closed container. Then, the base material having the first thin film formed thereon and being placed in the closed container, and a second source material for forming an inorganic solid electrolyte and being placed in a closed container are placed into a chamber space, which is substantially inactive to lithium and which is insulated from the air and provided adjacent to a second apparatus for forming a thin film. In the chamber space, the base material having the first thin film formed thereon and the second source material are respectively taken out from the closed containers. Then, the base material having the first thin film formed thereon and the second source material taken out are transferred into the second apparatus without being exposed to the air. In the second apparatus, the second source material is used, and a second thin film made of the second source material is formed on the first thin film. The base material having the first and second thin films formed thereon is transferred from the second apparatus without being exposed to the air into a chamber space, which is substantially inactive to lithium and which is insulated from the air and provided adjacent to the second apparatus. In the chamber space, the base material is placed into a closed container.
In the method, the first thin film may be formed by a vapor deposition method. The first thin film preferably has a thickness of 20 xcexcm or less. The thickness of the thin film formed is typically in the range of 0.1 xcexcm to 20 xcexcm, and preferably in the range of 1 xcexcm to 10 xcexcm.
As described above, the source materials, the base materials, and the base materials having the thin film can be handled without being exposed to air, so that a negative electrode for a lithium secondary cell can be prepared without being degraded by air.
In the above-described methods, when the source material is taken out and transferred into the apparatus, the chamber space and the apparatus are preferably filled with a gas selected from the group consisting of helium, nitrogen, neon, argon, krypton, a mixture gas of two or more from the foregoing, and dry air having a dew point of xe2x88x9250xc2x0 C. or below. When the base material having the thin film formed thereon is taken out from the apparatus and transferred into the chamber space to be placed into the closed container, the chamber space and the apparatus are also preferably filled with a gas selected from the group consisting of helium, nitrogen, neon, argon, krypton, a mixture gas of two or more from the foregoing, and dry air having a dew point of xe2x88x9250xc2x0 C. or below.
The inorganic solid electrolytes may include sulfides, oxides, nitrides, and mixtures thereof such as oxynitrides and oxysulfides. The sulfides may include Li2S, a compound of Li2S and SiS2, a compound of Li2S and GeS2, and a compound of Li2S and Ga2S3. The oxynitrides may include Li3PO4-xN2x/3, Li4SiO4-xN2x/3, Li4GeO4-xN2x/3 (0 less than x less than 4), and Li3BO3-xN2x/3 (0 less than x less than 3).
In the present invention, the thin film made of the inorganic solid electrolyte specifically contains components A to C as follows:
A: lithium, the content of which is in the range of 30% to 65% by atomic percent;
B: one or more elements selected from the group consisting of phosphorus, silicon, boron, germanium, and gallium; and
C: sulfur.
The thin film made of the inorganic solid electrolyte may further contain at least one of oxygen and nitrogen. The content of element B is typically 0.1% to 30% by atomic percent. The content of element C is typically 20% to 60% by atomic percent. The content of one or both of oxygen and nitrogen is typically 0.1% to 10%.
In the present invention, the thin film made of the inorganic solid electrolyte may be amorphous. The thin film made of the inorganic solid electrolyte preferably has an ionic conductance (conductivity) of at least 1xc3x9710xe2x88x924 S/cm at 25xc2x0 C. The ionic conductance of the thin film of the inorganic solid electrolyte at 25xc2x0 C. may be typically in the range of 1xc3x9710xe2x88x924 S/cm to 2.5xc3x9710xe2x88x923 S/cm, and preferably in the range of 5xc3x9710xe2x88x924 S/cm to 2.5xc3x9710xe2x88x923 S/cm. The thin film of the inorganic solid electrolyte formed in the present invention may have an activation energy of 40 kJ/mol or below. The activation energy of the thin film of the inorganic solid electrolyte may be in the range of 30 kJ/mol to 40 kJ/mol.
In the present invention, the thin film made of the inorganic solid electrolyte may be formed by a vapor deposition method, and typically, is formed by any one method selected from the group consisting of sputtering, vapor evaporation, laser ablation, and ion plating. In the present invention, the thin film made of lithium metal or a lithium alloy may also be formed by a vapor deposition method, and typically, is formed by any one method selected from the group consisting of sputtering, vapor evaporation, laser ablation, and ion plating.
The negative electrode produced by the present invention may be used to form a lithium secondary cell together with other necessary components such as a separator of porous polymer, a positive electrode, and an organic solution of electrolytes.
According to the present invention, the thin film made of the organic solid electrolyte is formed on the base material having a surface made of lithium metal or a lithium alloy, or the thin film made of lithium metal or a lithium alloy is formed on the negative electrode base material and then the thin film made of the inorganic solid electrolyte is formed thereon. The additive elements of the lithium alloys may include In, Ti, Zn, Bi, and Sn.
The base material having a surface made of lithium or a lithium alloy may be composed of a base material made of a metal or an alloy and a thin film made of lithium or a lithium alloy formed thereon. Specifically, the base material may be composed of a metal material (typically a metal foil or leaf) of at least one selected from the group consisting of copper, nickel, aluminum, iron, niobium, titanium, tungsten, indium, molybdenum, magnesium, gold, silver, platinum, alloys of two or more metals from the foregoing, and stainless steel, and a thin film made of lithium or a lithium alloy formed on the metal material. Alternatively, the base material for use in the process may be composed of a metal oxide such as SnO2 or an electrically conductive carbon such as graphite, and a thin film made of lithium or a lithium alloy formed thereon. In the above-described base materials, the thin film made of lithium or a lithium alloy typically has a thickness of 0.1 xcexcm to 20 xcexcm, and preferably a thickness of 1 xcexcm to 10 xcexcm. On the other hand, a foil or leaf made of lithium or a lithium alloy may be used as the base material. The base material used in the present invention may have a thickness of 1 xcexcm to 100 xcexcm from the viewpoint of application to the lithium cell and may have a thickness of 1 xcexcm to 20 xcexcm to give a compact product.
In the present invention, the negative electrode base material for use in depositing the thin lithium metal or lithium alloy film may be made of a metal, an alloy, a metal oxide such as SnO2, an electrically conductive carbon such as graphite, or the like. The metal and the alloy used for the base material may include at least one of copper, nickel, aluminum, iron, niobium, titanium, tungsten, indium, molybdenum, magnesium, gold, silver, platinum, or an alloy of two or more metals from the foregoing, or stainless steel. The negative electrode base material preferably has a thickness of not more than 100 xcexcm in order to reduce the size of the lithium cell, and preferably has a thickness of not less than 1 xcexcm in order to keep an enough strength of the base material. Therefore, the thickness of the negative electrode base material may be 1 xcexcm to 100 xcexcm, and may be 1 xcexcm to 20 xcexcm for compactness.
In the step of forming the thin film made of the inorganic solid electrolyte, the thin film made of the inorganic solid electrolyte may be formed on a heated base material by a vapor deposition method, or the thin film made of the inorganic solid electrolyte may be formed on a base material at room temperature or at a temperature below 40xc2x0 C. and then the thin film made of the inorganic solid electrolyte may be subjected to heat treatment. Such heat treatment allows the thin film to have a relatively high ionic conductance. Generally, a heater may be used for the heat treatment. The heater employed may be attached to a holder for holding the base material or may be a radiation heater. The heater heats the base material or the thin film formed on the base material. On the other hand, the heating may be effected through a temperature rise caused by plasma or the like during the film deposition. In the film deposition process, plasma or the like can heat the base material, so that the thin film can be formed on the base material having a increased temperature. The heat treatment can effectively be carried out at a temperature higher than room temperature (5xc2x0 C. to 35xc2x0 C.) or at a temperature of 40xc2x0 C. or higher. Thus, a temperature higher than room temperature such as a temperature of 40xc2x0 C. or higher, preferably 100xc2x0 C. or higher may be used as the base material temperature in the case that the thin film is heated through the heating of the base material, or as the temperature for the heat treatment of the formed thin film. The thin film of the inorganic solid electrolyte is generally amorphous, and specifically glassy. Therefore, when the heating temperature is too high and close to the glass transition temperature of the thin film of the inorganic solid electrolyte, the amorphous structure of the obtained thin film may be degraded, and its ionic conductance may be lowered. Thus, the heating temperature is preferably below the glass transition temperature of the thin film of the inorganic solid electrolyte. Based on this point, a temperature of 200xc2x0 C. or below is preferably used as the temperature of the substrate in the case that the thin film is heated through the heating of the substrate, or as the temperature for the heat treatment of the formed thin film. In addition, when the thin film of the inorganic solid electrolyte is formed on lithium metal, the heating temperature is preferably lower than 179xc2x0 C. which is the melting point of metal lithium. Thus, the heating temperature is preferably lower than a temperature at which the texture of the thin film of the inorganic solid electrolyte changes (for instance, the glass transition temperature of the thin film of the inorganic solid electrolyte) and lower than a temperature at which the structure of the base material can no longer be maintained (for instance, the melting point of the base material). Specifically, the heating temperature is preferably 40xc2x0 C. to 200xc2x0 C., and more preferably not lower than 100xc2x0 C. and lower than 179xc2x0 C.
The thin film of the inorganic solid electrolyte formed in the present invention typically has a thickness of 0.01 xcexcm to 10 xcexcm, and preferably a thickness of 0.1 xcexcm to 2 xcexcm.
In the present invention, the degree of vacuum of the background in the vapor deposition method is preferably not higher than 1.33xc3x9710xe2x88x924 Pa (1xc3x9710xe2x88x926 Torr). When the thin film of the inorganic solid electrolyte is formed on lithium metal or a lithium alloy, a low vacuum degree may induce oxidation or degradation of the lithium by water. The atmosphere under which the thin film is formed by the vapor deposition method may comprise a gas inactive to lithium, such as helium, neon, argon, krypton, or a mixture gas of two or more from the foregoing. The purity of the gas constituting the atmosphere is preferably at least 99.99% so that no degradation of the lithium due to the water may occur when the thin film of the inorganic solid electrolyte is formed on lithium metal or a lithium alloy.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.